This invention relates to an improved pulse arc welding apparatus in which a peak current and a base current are alternately provided to a welding unit.
Pulse arc welding apparatuses are widely known, as exemplified by that disclosed in commonly assigned application Ser. No. 247,451 filed on Mar. 25, 1981.
Referring to FIG. 1, which illustrates a typical conventional pulse are welding apparatus, a first D.C. voltage source 1 has a switching element 2, such as a transistor, connected to one of its output terminals. A first reactor 3 is connected between the transistor and a welding unit 13 through the input terminal 12. A current detector 4 is connected between another output terminal of the source 1 and input terminal 12', and detects the welding current in the unit 13 to operate a control circuit 6. The control circuit also receives set signals from upper and lower peak current limiters 7, 8, and in response to these inputs develops on-off switching signals for the transistor to thereby supply peak current pulses to the welding unit. A second D.C. voltage source 9 supplies a base current to the welding unit 13 through a resistor 10 and a second reactor 11.
Referring to the welding current waveform illustrated in FIG. 2, during peak current period T.sub.1 the transistor 2 is turned on to supply a peak current pulse from the source 1 to the welding unit 13 through the transistor and the reactor 3, and this peak current is always sensed by the current detector 4 and monitored by the control circuit 6. When the monitored current value reaches the setpoint I.sub.p1 of the upper limiter 7 the control circuit outputs a signal which turns off the transistor, whereafter the electromagnetic energy stored in the reactor 3 is dissipated through the path including the reactor, the welding unit 13, the current detector 4 and the diode 5, and the peak current is gradually decreased. When the monitored current value reaches the setpoint I.sub.p2 of the lower limiter 8, the control circuit 6 outputs a signal which turns on the transistor 2, whereby the peak current is increased again and such cyclic operations are repeated during the period T.sub.1. The peak current supplied to the welding unit 13 during T.sub.1 will thus be within the two values pre-set by the upper and lower peak current limiters 7, 8.
In the subsequent base current period T.sub.2 when the transistor 2 is held off, a base current much lower than the peak current is supplied to the welding unit 13 from the second D.C source 9 through the resistor 10 and the second reactor 11.
The second reactor 11 is essential to stabilize the welding performance by sustaining the arc during abrupt fluctuations in the load current. Furthermore, the greater the difference between I.sub.p1 and I.sub.p2 the lower the switching frequency of the transistor 2, and vice versa; a high switching frequency results in losses in and damage to the transistor caused by the abnormal heat generated. Consequently, the switching frequency should usually be less than 2 KHz.
Also, in this conventional pulse arc welding apparatus two D.C. voltage sources 1 and 9, the resistor 10 and two reactors 3 and 11 are required, and hence the cost of the apparatus is high and its physical size is unduly large. Although it has been proposed to use the first D.C. source 1 to supply both the peak and base currents, this has the disadvantage that a considerably large capacity resistor 10 is required which causes substantial heat loss and unfavorably affects the operation of the welding unit.